This invention relates to an improved incremental measuring system of the type having a grid scale, an electronic counter, a plurality of fixed reference marks disposed along the grid scale, and means for generating control signals for controlling the electronic counter in response to a freely selectable subset of the fixed reference marks.
In such measuring systems, the control signals generated in response to the selected reference marks can be used in various manners. For example, such control signals can be used to set the zero position of the electronic counter, to load preselected constant values into the counter at the commencement of the measurement procedure, and for the control or monitoring of interference impulses.
In one type of known incremental measuring instrument, a number of fixed reference marks are applied to the scale during the production of the scale and are fixed in accordance with the requirements of the user at that time. In this type of measuring system, it is not possible to change the number or position of either the reference marks or the associated control impulses.
In order to overcome this drawback of the prior art, it has been proposed in U.S. Pat. No. 4,101,764 (which is assigned to the assignee of the present application) to provide at least one switching means on the scale itself or immediately adjacent thereto. As disclosed more fully in U.S. Pat. No. 4,101,764, the switching means can be used to select one or more of the fixed reference marks to control an electronic counter. In the structure disclosed in the aforementioned patent, a magnet is associated with each of the fixed reference marks that is selected for operation. When the scanning unit moves past the magnet, the magnet actuates an electrical switch included in the scanning unit. An electronic logic component is included in the scanning unit which is responsive both to the electrical switch and to sensor means for sensing the fixed reference marks. This logic circuit produces an impulse control signal for control of the counter only when an electrical signal is provided both by the sensing means and by the magnetically controlled switch of the scanning unit.